1. Field of the Invention
The present invention relates to secrecy devices and more particularly, to a secrecy device which has a function of detecting the presence of a connection of a wiretapping telephone set to two communicating telephone sets.
2. Description of the Related Art
Conventionally, there have been suggested various types of secrecy devices for preventing telephone wiretapping. Among these secrecy devices, the secrecy device employing a time axis companding system is most common. The secrecy device of the time axis companding system is arranged so that a voice signal is subjected to such a processing (modulation) as to expand or compress the time axis of waveform of the voice signal in accordance with a predetermined algorithm, whereby, even when a wiretapper tries to listen to the voice signal with use of a common telephone set, he cannot understand the voice signal and merely can hear it as a noise because of the modulated voice signal. There are previously prepared a plurality of sorts of algorithms for companding the time axis of the voice signal which are associated with a plurality of predetermined secret codes respectively. The communicating two parties previously select one of the secrecy codes to realize a confidential conversation based on the algorithm of the selected secrecy code.
In this case, since the telephone receiver can know, on the basis of the secrecy code, the algorithm according to which the voice signal is modulated at the side of the telephone sender, the receiver can demodulate the modulated input voice signal to the original voice signal that can be normally understood by the receiver, thus realizing the confidential conversation.
However, employment of a higher level of modulation algorithm for a secrecy device generally requires the secrecy device to become more expensive. For this reason, conventional secrecy devices employing modulation systems other than the aforementioned time-axis companding system have been practically disadvantageous from the viewpoint of manufacturing cost. Thus most conventional secrecy devices employ the time axis companding system.
The prior art secrecy device of the time axis companding system, however, has had such a problem that, since the security of the confidential conversation depends merely on the secrecy code, if a wiretapper patiently tries to know the secrecy code by some means or other, then he might be able to know it finally, in which case he can easily wiretap the conversation.
Another big problem of the prior art secrecy device is that, even when the conversation between the two parties is wiretapped, they cannot recognize the fact and even cannot locate the information leakage route.